FIG. 1 generally illustrates a coherent optical receiver known in the art. As may be seen in FIG. 1, an inbound optical signal is received through an optical link 2, split into orthogonal received polarizations by a Polarization Beam Splitter 4, and then mixed with a Local Oscillator (LO) signal 6 by a conventional 90° optical hybrid 8. It should be noted that the received polarizations appearing at the output of the beam Splitter 4 will not normally correspond with the transmittal polarizations of the optical signal. The composite optical signals 10 emerging from the optical hybrid 8 are supplied to respective photodetectors 12, which generate corresponding analog electrical signals 14. The photodetector signals 14 are sampled by respective Analog-to-Digital (A/D) converters 16 to yield raw digital signals 18 which, in the illustrated example, correspond to In-phase (I) and Quadrature (Q) components of each of the received polarizations. Although one photodetector 12 is shown for each A/D 16 in FIG. 1, in some known systems a pair of photodetectors 12 may be used. From the A/D converter 16 block, the digital signals 18 are supplied to a digital signal processing block 20 for digital data recovery.
As may be appreciated, the resolution of the A/D converters 16 is a balance between performance and cost. In many cases, the resolution of the A/D converters 16 is matched to the encoding scheme of the received optical signal. Thus, for example, one or two-bits resolution may be used for detecting bi-state signals, such as On-Off-Keying (OOK) or phase shift keying (PSK) encoded signals. Multi-level encoded signals may be detected using corresponding multi-bit A/D converters. In some cases, multi-bit A/D converters are also used to enable improved accuracy of downstream digital processing of the raw digital signals 18 by the data recovery block 20. The sample rate of the A/D converters 16 is normally selected to match the symbol rate of the received optical signal. However, over-sampling of the optical signal is also known. For example, in some systems, the sample rate may be selected to satisfy the Nyquist criterion for the highest anticipated symbol rate of the received optical signal. For the case of a 10Gbaud optical communications system, this represents a sample rate of about 20GHz.
Typically, the A/D converters 16 are controlled by a master clock signal (not shown in FIG. 1) which governs both the timing with which the received analog signal is measured and the timing with which a corresponding digital value is latched out of the A/D. Typically, the timing with which the analog signal is measured (which may be referred to as the sample timing) is adjusted so as to obtain a desired nominal sample phase relative a selected characteristic of the analog signal. For example, in the case of a received optical signal, it is frequently desirable to select the nominal sample phase to correspond with the center of the eye opening.
Typically, the precise timing with which digital values are latched out of the A/D converter is of less concern, except in cases in which two or more parallel A/D converters are used to sample a common analog signal. In such cases it us usually desired that the digital values of one digital signal have some desired phase relationship with the digital values of an adjacent digital signal. A common example of this is shown in the receiver of FIG. 1, in which a respective pair of parallel A/D converters are used to sample In-Phase and Quadrature components of each received polarization of the optical signal. For each polarization, the I and Q signal components are separated in the optical hybrid 8, so the sample timing required to obtain a desired nominal sample phase will be identical for both A/D converters. Similarly, it is desired that the corresponding digital values of both digital signal (that is, digital values derived from analog signal measurements taken at the same sample timing) are latched out of the respective A/D converters phase aligned, so that a common clock can be used to latch digital values out of both A/D converters and into downstream digital signal processing circuitry.
Optical signals received through conventional optical links are typically distorted by significant amounts of chromatic dispersion (CD) and polarization dependent impairments such as Polarization Mode Dispersion (PMD), polarization angle changes and polarization dependent loss (PDL). Chromatic dispersion (CD) on the order of 30,000 ps/nm, and polarization rotation transients at rates of 105 Hz are commonly encountered.
Various methods and systems intended to address some of these limitations are known in the art. For example, a method of compensating polarization angle impairments are described in PLL-Free Synchronous QPSK Polarization Multipex/Diversity Receiver Concept with Digital I&Q Baseband Processing, R Noé, IEEE Photonics Technology Letters, Vol. 17, No. 4, April 2005.
Applicant's co-pending U.S. patent applications Nos. 11/294,613 filed Dec. 6, 2005 and entitled “Polarization Compensation In A Coherent Optical Receiver”; 11/315,342 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Dispersion Impairments”; 11/315,345 filed Dec. 23, 2005 and entitled “Clock Recovery From An Optical Signal With Polarization Impairments”; 11/366,392 filed Mar. 2, 2006 and entitled “Carrier Recovery In A Coherent Optical Receiver”; and 11/423,822 filed Jun. 13, 2006 and entitled “Signal Acquisition In A Coherent Optical Receiver”, the content of all of which are hereby incorporated herein by reference, describe methods and systems of reliable signal acquisition, clock recovery and polarization compensation in the presence of moderate-to-severe optical impairments of a received optical signal.
As may be appreciated, accurate compensation of polarization angle impairments in the manner proposed by Noé [supra], and in the systems described in the above-mentioned Applicant's co-pending applications, requires that the digital signals generated by the analog-to-digital (A/D) converters 16 accurately reproduce the amplitude and/or phase information of the original optical signal received through the optical link 2. However, in real-world network systems, design limitations and manufacturing variations in the optical hybrid 8 prevents perfect phase alignment between the optical signals 10 supplied to the photodetectors 12. Additional errors accumulate within the photodetectors 12 and the A/D converters 16, due to differing signal propagation delays, and different amplitude and phase responses of the involved components. In a receiver utilizing multiple parallel A/D converters, these errors result in a difference between the desired nominal sample timing and the actual sample timing of each A/D converter. These sample timing differences mean that there will be corresponding differences in the actual sample phase of each A/D converter, relative the optical signal arriving at the polarization beam splitter 4. This sample phase differential may appear in the digital signals 18 as a combination of additive noise, distortion, polarization or phase degradations, and so impair the performance of the digital receiver.
Methods of achieving alignment between parallel digital signals output from parallel A/D converters are well known. However, these methods are focused on compensating differences in the timing of the digital values latched out of the parallel A/D converters. These methods do not address the issue of sample timing (and thus sample phase) errors embedded within each digital signal, which will be present even when corresponding values of each digital signal are properly aligned. At low symbol line rates, the embedded sample timing errors will be very much less than the symbol period, and thus their impact will be relatively minor. On the other hand, real-world optical communications network commonly operate at line rates of 10Gbaud or higher. At such high symbol rates, sample timing errors embedded within the digital signals can easily be several symbol periods.
It is well known how to use frame information to align the timing of bit streams, or demultiplexed bit streams, from parallel framed binary optical signals. This problem is quite distinct from the alignment of a plurality of multi-bit samples of one optical signal. Furthermore, as the multi-bit samples are generally not decoded binary symbols, but rather are often relatively unequalized samples of an analog signal, these framing methods are not applicable.
In addition to the above-described difficulties associated with sample timing errors between the parallel digital signals 18, timing errors can also impact each individual digital signal. For example, the systems described in the above-mentioned Applicant's co-pending applications utilize Nyquist sampling of the optical signal using multi-bit A/D converters. In a 10Gbaud optical communications system, this represents a sample rate of about 20GHz. An A/D converter resolution of at least 5 bits is preferred, in order to enable accurate compensation of impairments. This combination of high resolution and high sample rate is difficult to achieve in a single A/D converter, at reasonable cost.
A possible solution to this problem is to utilize several lower-speed A/D multi-bit converters in parallel, as shown in FIG. 2. Thus, a distribution network 22 supplies the photodetector current 14 to a respective track-and-hold circuit 24 of each one of N sub-channel A/D converters 26. Each sub-channel A/D converter 26 is a multi-bit A/D converter operating at 1/Nth of the aggregate sample rate to generate a respective sub-channel digital signal 28. By suitably setting the phase offset of the respective sample clock of each sub-channel A/D converter 26, the photodetector signal 14 will be sampled at evenly spaced intervals corresponding to the desired aggregate sample rate. An interleaver 30 operates in a known manner to combine the N sub-channel digital signals 28, in the correct sequence, to produce the desired digital signal 18. As will be appreciated, this arrangement relies on each sub-channel A/D converter 26 measuring the photodetector signal 14 with the correct sample timing within a very narrow tolerance. An error in the average sample phase of any one sub-channel A/D converter will appear as noise in the output digital signal 18. Since average sample phase is a function of manufacturing variations, temperature, and component aging, all of the sub-channel A/D converters 26 will normally exhibit at least some sample phase error, plus clock jitter. While the amount of degradation due to sub-channel sample timing error can be characterised mathematically, there is no simple means of compensating this degradation by processing the digital signal 18 downstream of the A/D converter 16.
In a controlled environment, such as a factory or laboratory, timing errors in an A/D converter can be measured with the use of a known electrical test signal. A single sine wave is commonly used for this purpose. This allows the errors that are present in the A/D converter at that time and temperature to be measured. However, especially when in operation in an installed “real-world” network, an optical receiver does not received a precise test signal. The received signal generally contains apparently random scrambled data, distorted by variable amounts of dispersion and polarization effects, and degraded by noise. These are not suitable A/D test signals for use with the known methods.
The above described issues of prior art A/D converters have been described in the context of a coherent optical receiver. However, it will be appreciated that these same issues will arise in any context in which it is desired to maintain accurate sample phase alignment between two or more parallel A/D converters.
Accordingly, cost-effective techniques for controlling sample phase alignment in an optical receiver remain highly desirable.